Biological Journal of the Linnean Society ( 1984) 21: 243-25 1. With 2 figures
Relationships of the Galapagos flora
DUNCAN M. PORTER
Department of Biology, Virginia Polytechnic Institute &
State University, Blacksburg, Virginia 24061, U.S.A .
Joseph Dalton Hooker’s pioneer 1847 paper on Galapagos plants and their relationships is a classic
in the field of phytogeography. I t was the first study of its kind to be published, comparing the
islands’ flora with island and continental floras elsewhere, hypothesizing on the dispersal
mechanisms of the plants, and pointing out anomalies in the inter-island distributions of the native
species. These are still three of the primary concerns of contemporary phytogeographers, and the
present paper contrasts Hooker’s findings with those of today. Despite the accumulation of a large
amount of data since his time, many of Hooker’s conclusions regarding Galapagos phytogeography
remain valid.
KEY WORDS:-Island
Darwin.
biogeography
-
Galapagos Islands
-
Joseph Dalton Hooker
-
Charles
CONTENTS
Introduction . . . .
Phytogeographic relationships
Dispersal . . . . .
Inter-island relationships .
Acknowledgements
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References.
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243
244
247
250
250
250
INTRODUCTION
Our knowledge of the vascular plants of the Galapagos Islands and their
relationships has, not surprisingly, markedly increased since Charles Darwin’s
six-week visit on H.M.S. Beagle in 1835. Darwin was not the first to collect
plants in the archipelago, having been preceded by the British naturalists
Archibald Menzies in 1795, David Douglas, Dr John Scouler and James
McRae in 1825, and Hugh Cuming in 1829. The first flora of the islands
(Hooker, 1847a) was based on their collections, except for those of Menzies
which were labelled “Sandwich Islds.”, and a few others. Of these collections,
Darwin’s was by far the largest, not being surpassed until the California
Academy of Science’s year-and-a-day expedition in 1905-1 906. Darwin’s plant
collections also are the most important for typification of Galapagos endemics
(Porter, 1980b).
Although he played down his prowess as a plant collector while on the Beagle,
Darwin did admit to his mentor the Rev. John Stevens Henslow, Professor of
Botany in the University of Cambridge, that he had collected everything that he
0024-4066/84/010243
+ 09 S03.00jO
243
01984 The Linnean Society of London
244
D. M. PORTER
saw in flower in the islands. T h e 210 collections of 173 taxa that Darwin made
in 1835 represent a sample of about 24% of the presently known flora.
PHYTOGEOGRAPHIC RELATIONSHIPS
Not only did Joseph Dalton Hooker write the first floristic study of the
Galapagos Islands, he soon produced the first phytogeographic study as well
(Hooker, 1847b). As with the former, the latter was also primarily based on
Darwin’s collections. After making the point that about half of the species are
endemic, Hooker (184713: 235-236) continued as follows.
The results of my examination have been, that the relationship of the Flora to that of
the adjacent continent is a double one, the peculiar or new species being for the most
part allied to plants of the cooler parts of America, or the uplands of the tropical
latitudes, whilst the non-peculiar are the same as abound chiefly in the hot and
damper regions, as the West Indian islands and the shores of the Gulf of Mexico; also
that, as is the case with the Fauna, many of the species, and these the most
remarkable, are confined to one islet of the group, and often represented in others by
similar, but specifically very distinct congeners.
A few pages later in the paper, Hooker (1847b : 239) slightly modified his
remarks on the relationships of the flora.
Here, as in other countries, the vegetation is formed of two classes of plants; the one
peculiar to the group, the other identical with what are found elsewhere. In this there
are even indications of the presence of two nearly equal Floras, an indigenous and
introduced, and these of a somewhat different stamp; for the introduced species are
for the most part the plants of the West Indian islands and of the lower hot parts of
the South American coast; whilst the peculiar Flora is chiefly made up of species not
allied to the introduced, but to the vegetation which occurs in the Cordillera or the
extra-tropical parts of South America.
Here, Hooker is using “indigenous” or “peculiar” for endemic, a term not
widely used in a geographical context until a few years later (DeCandolle,
1855). H e uses “introduced” to mean non-endemic, rather than those species
introduced by human activity.
A further clarification is made in his introduction to a more detailed
discussion of the archipelago’s phytogeographical relationships (Hooker,
1847b: 250).
In this second part of the essay I propose to treat of the Flora of the Galapagos as
divisible into two types: these are the West Indian (including Panama), to which the
plants common to other countries and the dubious species almost universally belong;
and the Mexican and temperate American type, or that under which the great
majority of the peculiar species will rank.
These relationships, recognized by Hooker o n the basis of about 25% of
today’s known vascular plant flora, still hold. T h e endemic plants have their
closest relatives primarily (56%) in the Andean region (Table 1 ) . I n addition,
52% of the original introductions that have given rise to the present endemic
flora presumably have come from the Andean region. T h e Neotropical region,
which overlaps with much of the Andean region, accounts for 27% of endemic
taxa and of introductions. T h e third-largest element is the endemics with
Pantropical affinities. Hooker did not think that many species with Pantropical
relationships were represented in the islands, clearly because of a lack of
knowledge of the archipelago’s entire flora and of these species’ distributions
10 (9%)
61 (27%)
31 (27%)
3
4
54
2
I
15
18 (8%)
Pantropical
60 (52%)
1
9
118
128 (56%)
Andean
5 (4%)
4
6 (3%)
2
Mexico &
C. America
3
3 (1%)
3 (3%)
6 (5%)
1(1%)
North America
4 (2%)
Caribbean
1
8
9 (4%)
4
South America
116
8
19
202
229
Total
Geographical areas are defined as follows: Neotropical (distributed generally in the American tropics); Puntropical (distributed in both the Old and New World
tropics); Andean (occurring only in western South America from Venezuela to Chile, generally or in part); Mexico and Central America (occurring only in Mexico
and/or Central America, and in one case also in northern Colombia and Venezuela); South America (occurring only in extra-Andean South America); Caribbean
(occurring in the West Indies and often also on the edges of the surrounding continents); North America (occurring in the south-western United States and ajacent
northern Mexico).
Pteridophytes
Monocotyledons
Dicotyledons
Total
Number of single
original
introductions
from each area
Neotropical
Table 1. Geographical relationships of the endemic vascular plants of the Galapagos Islands
8
D. M. PORTER
246
elsewhere. Those that were present he thought to be derived from South
America, not from the westward. Pantropical relatives apparently provided 9%
of the original introductions, which evolved into 8% of the presently known
endemics.
The indigenous non-endemics have their greatest numbers occurring
elsewhere in the Neotropics (Table 2). Here are to be found 48% of the
indigenes; 25% are Pantropical, and 21 yo Andean. The Neotropical,
Pantropical, and Andean groups all overlap in adjacent South America, so it is
possible that 88% of the introductions that have given rise to the endemic flora,
and 94% of introductions of indigenes, have come from this nearby land mass.
The importance of the Pantropical element in the Galapagos flora was not
recognized until the publication of the second floristic study by N. J. Anderson
(1855). Andersson’s work was based on his own collections, made during a 10day visit to the islands in 1852. Interestingly, the first to indicate the flora’s
relationship to adjacent South America was Charles Darwin (1839). Th’is was
done before Hooker had seen Darwin’s collections, although Henslow had
examined them. However, this insight came about through Darwin’s own field
observations (see Barlow, 1933), and not from information supplied by Henslow.
Henslow did a superb job in handling Darwin’s collections, shipped to him from
South America, but he failed in his promise to identify the plants (Porter,
1980a).
Subsequent studies on Galapagos phytogeography (Robinson & Greenman,
1895; Robinson, 1902; Stewart, 191 1, 1915; Svenson, 1935, 1942, 1946; van
Balgooy, 1960; Porter, 1976, 1979, 1983, in press) have continued to provide
evidence for Hooker’s hypotheses of relationship. The close relationship of the
vascular flora with that of adjacent South America was first documented by
Svenson (1935, 1942, 1946), and is supported by my own research (Porter,
1976, 1979, 1983, in press).
Hooker’s estimate of about 50% endemism still holds if all subgeneric taxa are
considered. Given 54 1 indigenous species, subspecies, and varieties, 229
endemics and 312 indigenes, endemism would be 42%. However, at the species
level, this drops to 34%, 170 endemic species out of a total native flora of 497
species. If flowering plants only are considered, species endemism is 41 yo, while
that for species, subspecies, and varieties rises to 51%. The relationships of each
endemic taxon are discussed in detail elsewhere (Porter, 1979, 1983), as are
those of the entire flora (Porter, 1983).
The first section of Hooker’s paper concludes with a family-by-family
discussion of geographical relationships. In it he makes the observation that the
Table 2. Geographical relationships of the indigenous, non-endemic vascular plants
of the Galapagos Islands
Neotropical
Pteridophytes
Monocotyledons
Dicotyledons
Total
Percentage
52
35
63
150
48%
Pantropical
18
24
35
77
25%
Andean
22
5
39
66
21%
Mexico &
C. America
1
2
3
1%
Caribbean
6
4
6
16
5%
Total
99
68
145
312
247
RELATIONSHIPS OF THE GALAPAGOS FLORA
flora is basically a disharmonic one. That is, one comprised of a skewed sample
of easily dispersed taxa and not of a random sample of all taxa in the flora of the
adjacent continental area:
the more an island is indebted to a neighbouring continent for its vegetation, the
more fragmentary does its flora appear, migration being effected by the transport of
isolated individuals, generally in no wise related, while an independent flora is
generally made up of groups, the lowest order of which we call genera. (Hooker,
1847b : 247).
The Galapagos flora is indeed disharmonic, as is the fauna, providing further
biological evidence that it has been derived by long-distance dispersal.
DISPERSAL
Hooker’s paper was not only a pioneering study in the geographical
relationships of island floras, it also was the first to speculate on how such a flora
might be derived. Several pages were devoted to a family-by-family discussion of
adaptations for dispersal. Hooker (1847b : 253) concluded that “The means of
transport which may have introduced these plants are, oceanic and aerial
currents, the passage of birds, and man”. Subsequent botanists (Andersson,
1855; Robinson, 1902; Stewart, 191 1; Svenson, 1942) added comments on
dispersal mechanisms, but none provided as detailed observations as those of
Hooker.
More recently, in a study based on Stewart’s (191 1) flora, Carlquist (1967)
determined that the flowering plants were derived as follows: 73% by bird
dispersal, 23% by oceanic drift, and 4% by wind currents. With the publication
of a modern flora for the islands (Wiggins & Porter, 1971), it became possible to
more accurately portray the relationships of the plants and their methods of
dispersal. For the vascular plants, this revealed that 40% had been derived
through birds, 32% by human carriage, 21% by wind, and 6% through drift
(Porter, 1976). Natural means of dispersal were 60% by birds, 31 yo wind, and
9% drift.
Since the publication of the Flora of the Galapagos Islands, a series of papers
have appeared, adding to and subtracting from the flora (see Schofield, 1973,
1980, for references). Addition of this information does not significantly change
my previously calculated figures (Table 3).
If only the flowering plants are calculated, then the overwhelming
importance of bird dispersal for this group is revealed. Of the total successful
introductions that have given rise to the presently known angiosperms, 48%
have been by birds, 39% by humans, 7% by drift, and 6% by wind. However, if
Table 3. Original introductions that have resulted in the present vascular plant
flora of the Galapagos Islands
Birds
Pteridophytes
Monocotyledons
Dicotyledons
Total
Total for
natural
introductions
Humans
1
64
I 78
243 (40%)
243 (59%)
39
156
195 (32%)
Wind
Oceanic Drift
Total
106
14
14
134 (22%)
3
33
36 (6%)
107
I20
38 1
608
134 (32%)
36 (9%)
413
248
D. M. PORTER
only natural agencies are considered, this changes to 79% of the introductions
having arrived on or in birds, 12% floating in on oceanic currents, and 9%
wafted on the wind. Taxon by taxon discussion of dispersal mechanisms is
published elsewhere (Porter, 1979, 1983).
Hooker was the first botanist, and the only one until recently, to recognize the
importance of human impact on the islands’ flora. He wrote that currents,
winds, and birds played their various roles in the derivation of the plants, then
added (Hooker, 184713 : 254).
M a n is the last agent to which I alluded: that he has been already active is very
perceptible from the fact, that Charles Island, the only colonized island, contains the
smallest proportion of peculiar plants, and numerically far the most of these common
to and probably introduced from the coast with cultivation.
So far as is known a t present, humans have introduced 195 weeds and escapes
from cultivation that reproduce themselves in the islands. Indeed, humans are
now the most important dispersers of plants, both to and within the archipelago.
For example, the Pantropical (originally tropical Asian) weed Cleome uiscosa was
first collected on the island of Baltra in 1963. Personal observation in 1977
showed it to be common in the vicinity of the Baltra airport, and in 1978 it
formed large populations across the island. I n 1981 I found it along the trail
above Tagus Cove on Isla Isabela (Fig. I ) . Tagus Cove is usually visited by
Figure 1. Cleome aiscosu (Capparidaceae), a pantropical weed introduced into the Galapagos Islands
through human activity. T h e fruits are thickly covered with sticky glandular hairs and readily stick
to clothing. It was first collected on Isla Baltra in 1963 and has since spread to Isla Isabela.
RELATIONSHIPS O F T H E GALAPAGOS FLORA
249
tourists soon after their arrival in the islands at the airport on Baltra. Obviously,
Cleome uiscosa, easily dispersed because of the many sticky hairs on the fruits, is
now a prime candidate for distribution throughout the islands through the
inadvertent intervention of humans.
A more alarming example is that of the endemic grass, Cenchrus platyacanthus.
Like other members of its genus, the fruits are enclosed in a burr-like involucre
that is provided with numerous retrorsely-barbed spines, which make them
easily dispersed. Presumably, they are naturally dispersed by birds. However, I
know by personal experience that these disseminules are as likely to be found
attached to one's trousers or socks as to the feathers of a bird (Fig. 2).
In 1978 I found a specimen of Cenchrus platyacanthus growing along a path on
Isla Plaza Sur, a small, well-collected island much visited by tourists, from
which it had not hitherto been reported. Thus, humans are not only introducing
new elements into the flora of the islands from elsewhere, they are interfering
with the distributions of the endemic plants as well. Their effects on the
vegetation, and those of their introduced animals, also are profound (see
Hamann, 1975; van der Werff, 1979).
Figure 2. Cenchrus plulyucanthus (Poaceae), an endemic grass that occurs on a number of islands in
the archipelago. The fruit is surrounded by a burr-like structure that aids in dispersal. Natural
dispersal is presumably by birds, but dispersal by humans is also possible. I n 1978 it was found
along a trail frequented by tourists on Isla Plaza Sur, a small, well-known island on which it had
not hitherto been seen.
250
D. M. PORTER
INTER-ISLAND RELATIONSHIPS
Another subject which fascinated Hooker was the apparently restricted ranges
of various species from island to island within the archipelago. He stated
(Hooker, 1847b : 239),
In the third place, I shall allude to the most singular feature in the botany of the
group, the unequal dispersion of the species, the restriction of most of them to one
islet, and the representation of others by allied species in two or more of the other
islets.
Hooker reckoned that 112 of 128 (88%) of his “peculiar” (that is, endemic)
taxa occurred on only a single island. Subsequent investigators, particularly
Robinson (1902) and Kroeber (1916), also have commented on the number of
endemics in the islands with restricted distributions.
This line of investigation begun by Hooker has been extended by several
recent studies, which attempted to discover correlations between species
numbers on the various islands and such parameters as island area, elevation,
distance to the nearest island, distance to the center of the archipelago, etc.
Island elevation, the area of the adjacent island, and the log of the island area
were found to be significant variables in predicting species numbers (Hamilton
et al., 1963; Johnson & Raven, 1973; Simpson, 1974). However, Connor &
Simberloff (1978) repeated the calculations of the former investigators and,
adding more precise information on island areas, elevations, and species
numbers, found that only area contributed significantly to explaining variance
in species numbers from island to island, a point made by Kroeber in 1916.
Connor & Simberloff (1978 :219) also showed that, “The number of botanical
collecting trips to each of the Galapagos islands is a better predictor of species
numbers than are area, elevation, or isolation”.
As knowledge of species’ distributions has advanced through further collecting
and observation, many of the single-island taxa found by Hooker have
disappeared as well. According to my latest calculations, no pteridophyte, two
monocotyledon, and 65 dicotyledon taxa (30% of the endemics) occur on only a
single island. A majority of these taxa of restricted distribution are subspecies or
varieties, and some are questionably distinct entities. These presumably are
recently evolved taxa, and they provide biological evidence for the hypothesis
that the islands themselves are geologically young (Porter, in press), which is
confirmed by geological evidence discussed in another paper at this symposium.
ACKNOWLEDGEMENTS
Field trips to the Galapagos Islands in 1977, 1978, and 1981 were made
courtesy of Harvard University’s Friends of the Museum of Comparative
Zoology. I was able to present the gist of this paper a t the Linnean Society
symposium “Evolution in the Galapagos Islands” through a travel grant from
the Society. Both organizations are gratefully acknowledged.
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